3) Expected result
Prior to my recent upgrade to Lucid (Ubuntu release 10.04), I was able to unmap the Lock modifier without problems. As a vim user I find it quite useful to map the [CapsLock] key to Escape, and prefer to disable the Lock modifier entirely: as I rarely write anything in all caps, I seldom have any use for it, and an accidental activation of it can cause minor confusion when editing files in Vim.
My previous ~/.xmodmap file used the "clear Lock" directive to disable the Lock modifier. Upon rebooting after upgrading to Lucid I discovered that this functionality was no longer activated, ie that the Caps Lock key continues to control the Lock modifier's state.
I expected it to be activated and for the Caps Lock key to not affect the Lock modifier state.

4) What happened instead:
The Caps Lock key continued to control the Lock modifier's state. The error message given above was output in response to an xmodmap command to clear or remove that functionality.

Also: please note that my exclusion of gdm debugging information was not so much due to privacy concerns as because I haven't run it in months. Although this install is an Ubuntu install, I installed the kubuntu-desktop metapackage some time ago, and so am using kdm rather than gdm. In any case this seems unlikely to be affecting xmodmap functionality.